/*
* Created by boil on 2023/2/17.
*/

#ifndef RENDU_CORE_CONTAINER_DENSE_MAP_H_
#define RENDU_CORE_CONTAINER_DENSE_MAP_H_

#include <cmath>
#include <cstddef>
#include <functional>
#include <iterator>
#include <limits>
#include <memory>
#include <tuple>
#include <type_traits>
#include <utility>
#include <vector>
#include "define/define.h"
#include "base/compressed_pair.h"
#include "base/iterator.h"
#include "base/memory.h"
#include "base/type_traits.h"
#include "fwd.h"

namespace rendu {

/**
 * @cond TURN_OFF_DOXYGEN
 * Internal details not to be documented.
 */

namespace internal {

template<typename Key, typename Type>
struct dense_map_node final {
  using value_type = std::pair<Key, Type>;

  template<typename... Args>
  dense_map_node(const std::size_t pos, Args &&...args)
      : next{pos},
        element{std::forward<Args>(args)...} {}

  template<typename Allocator, typename... Args>
  dense_map_node(std::allocator_arg_t, const Allocator &allocator, const std::size_t pos, Args &&...args)
      : next{pos},
        element{rendu::make_obj_using_allocator<value_type>(allocator, std::forward<Args>(args)...)} {}

  template<typename Allocator>
  dense_map_node(std::allocator_arg_t, const Allocator &allocator, const dense_map_node &other)
      : next{other.next},
        element{rendu::make_obj_using_allocator<value_type>(allocator, other.element)} {}

  template<typename Allocator>
  dense_map_node(std::allocator_arg_t, const Allocator &allocator, dense_map_node &&other)
      : next{other.next},
        element{rendu::make_obj_using_allocator<value_type>(allocator, std::move(other.element))} {}

  std::size_t next;
  value_type element;
};

template<typename It>
class dense_map_iterator final {
  template<typename>
  friend
  class dense_map_iterator;

  using first_type = decltype(std::as_const(std::declval<It>()->element.first));
  using second_type = decltype((std::declval<It>()->element.second));

 public:
  using value_type = std::pair<first_type, second_type>;
  using pointer = input_iterator_pointer<value_type>;
  using reference = value_type;
  using difference_type = std::ptrdiff_t;
  using iterator_category = std::input_iterator_tag;

  constexpr dense_map_iterator() noexcept
      : it{} {}

  constexpr dense_map_iterator(const It iter) noexcept
      : it{iter} {}

  template<typename Other, typename = std::enable_if_t<
      !std::is_same_v<It, Other> && std::is_constructible_v<It, Other>>>
  constexpr dense_map_iterator(const dense_map_iterator<Other> &other) noexcept
      : it{other.it} {}

  constexpr dense_map_iterator &operator++() noexcept {
    return ++it, *this;
  }

  constexpr dense_map_iterator operator++(int) noexcept {
    dense_map_iterator orig = *this;
    return ++(*this), orig;
  }

  constexpr dense_map_iterator &operator--() noexcept {
    return --it, *this;
  }

  constexpr dense_map_iterator operator--(int) noexcept {
    dense_map_iterator orig = *this;
    return operator--(), orig;
  }

  constexpr dense_map_iterator &operator+=(const difference_type value) noexcept {
    it += value;
    return *this;
  }

  constexpr dense_map_iterator operator+(const difference_type value) const noexcept {
    dense_map_iterator copy = *this;
    return (copy += value);
  }

  constexpr dense_map_iterator &operator-=(const difference_type value) noexcept {
    return (*this += -value);
  }

  constexpr dense_map_iterator operator-(const difference_type value) const noexcept {
    return (*this + -value);
  }

  [[nodiscard]] constexpr reference operator[](const difference_type value) const noexcept {
    return {it[value].element.first, it[value].element.second};
  }

  [[nodiscard]] constexpr pointer operator->() const noexcept {
    return operator*();
  }

  [[nodiscard]] constexpr reference operator*() const noexcept {
    return {it->element.first, it->element.second};
  }

  template<typename Lhs, typename Rhs>
  friend constexpr std::ptrdiff_t operator-(const dense_map_iterator<Lhs> &, const dense_map_iterator<Rhs> &) noexcept;

  template<typename Lhs, typename Rhs>
  friend constexpr bool operator==(const dense_map_iterator<Lhs> &, const dense_map_iterator<Rhs> &) noexcept;

  template<typename Lhs, typename Rhs>
  friend constexpr bool operator<(const dense_map_iterator<Lhs> &, const dense_map_iterator<Rhs> &) noexcept;

 private:
  It it;
};

template<typename Lhs, typename Rhs>
[[nodiscard]] constexpr std::ptrdiff_t operator-(const dense_map_iterator<Lhs> &lhs,
                                                 const dense_map_iterator<Rhs> &rhs) noexcept {
  return lhs.it - rhs.it;
}

template<typename Lhs, typename Rhs>
[[nodiscard]] constexpr bool operator==(const dense_map_iterator<Lhs> &lhs,
                                        const dense_map_iterator<Rhs> &rhs) noexcept {
  return lhs.it == rhs.it;
}

template<typename Lhs, typename Rhs>
[[nodiscard]] constexpr bool operator!=(const dense_map_iterator<Lhs> &lhs,
                                        const dense_map_iterator<Rhs> &rhs) noexcept {
  return !(lhs == rhs);
}

template<typename Lhs, typename Rhs>
[[nodiscard]] constexpr bool operator<(const dense_map_iterator<Lhs> &lhs,
                                       const dense_map_iterator<Rhs> &rhs) noexcept {
  return lhs.it < rhs.it;
}

template<typename Lhs, typename Rhs>
[[nodiscard]] constexpr bool operator>(const dense_map_iterator<Lhs> &lhs,
                                       const dense_map_iterator<Rhs> &rhs) noexcept {
  return rhs < lhs;
}

template<typename Lhs, typename Rhs>
[[nodiscard]] constexpr bool operator<=(const dense_map_iterator<Lhs> &lhs,
                                        const dense_map_iterator<Rhs> &rhs) noexcept {
  return !(lhs > rhs);
}

template<typename Lhs, typename Rhs>
[[nodiscard]] constexpr bool operator>=(const dense_map_iterator<Lhs> &lhs,
                                        const dense_map_iterator<Rhs> &rhs) noexcept {
  return !(lhs < rhs);
}

template<typename It>
class dense_map_local_iterator final {
  template<typename>
  friend
  class dense_map_local_iterator;

  using first_type = decltype(std::as_const(std::declval<It>()->element.first));
  using second_type = decltype((std::declval<It>()->element.second));

 public:
  using value_type = std::pair<first_type, second_type>;
  using pointer = input_iterator_pointer<value_type>;
  using reference = value_type;
  using difference_type = std::ptrdiff_t;
  using iterator_category = std::input_iterator_tag;

  constexpr dense_map_local_iterator() noexcept
      : it{},
        offset{} {}

  constexpr dense_map_local_iterator(It iter, const std::size_t pos) noexcept
      : it{iter},
        offset{pos} {}

  template<typename Other, typename = std::enable_if_t<
      !std::is_same_v<It, Other> && std::is_constructible_v<It, Other>>>
  constexpr dense_map_local_iterator(const dense_map_local_iterator<Other> &other) noexcept
      : it{other.it},
        offset{other.offset} {}

  constexpr dense_map_local_iterator &operator++() noexcept {
    return offset = it[offset].next, *this;
  }

  constexpr dense_map_local_iterator operator++(int) noexcept {
    dense_map_local_iterator orig = *this;
    return ++(*this), orig;
  }

  [[nodiscard]] constexpr pointer operator->() const noexcept {
    return operator*();
  }

  [[nodiscard]] constexpr reference operator*() const noexcept {
    return {it[offset].element.first, it[offset].element.second};
  }

  [[nodiscard]] constexpr std::size_t index() const noexcept {
    return offset;
  }

 private:
  It it;
  std::size_t offset;
};

template<typename Lhs, typename Rhs>
[[nodiscard]] constexpr bool operator==(const dense_map_local_iterator<Lhs> &lhs,
                                        const dense_map_local_iterator<Rhs> &rhs) noexcept {
  return lhs.index() == rhs.index();
}

template<typename Lhs, typename Rhs>
[[nodiscard]] constexpr bool operator!=(const dense_map_local_iterator<Lhs> &lhs,
                                        const dense_map_local_iterator<Rhs> &rhs) noexcept {
  return !(lhs == rhs);
}

} // namespace internal

/**
 * Internal details not to be documented.
 * @endcond
 */

/**
 * @brief Associative container for key-value pairs with unique keys.
 *
 * Internally, elements are organized into buckets. Which bucket an element is
 * placed into depends entirely on the hash of its key. Keys with the same hash
 * code appear in the same bucket.
 *
 * @tparam Key Key type of the associative container.
 * @tparam Type Mapped type of the associative container.
 * @tparam Hash Type of function to use to hash the keys.
 * @tparam KeyEqual Type of function to use to compare the keys for equality.
 * @tparam Allocator Type of allocator used to manage memory and elements.
 */
template<typename Key, typename Type, typename Hash, typename KeyEqual, typename Allocator>
class dense_map {
  static constexpr float default_threshold = 0.875f;
  static constexpr std::size_t minimum_capacity = 8u;

  using node_type = internal::dense_map_node<Key, Type>;
  using alloc_traits = std::allocator_traits<Allocator>;
  static_assert(std::is_same_v<typename alloc_traits::value_type, std::pair<const Key, Type>>, "Invalid value type");
  using sparse_container_type = std::vector<std::size_t, typename alloc_traits::template rebind_alloc<std::size_t>>;
  using packed_container_type = std::vector<node_type, typename alloc_traits::template rebind_alloc<node_type>>;

  template<typename Other>
  [[nodiscard]] std::size_t key_to_bucket(const Other &key) const noexcept {
    return fast_mod(static_cast<size_type>(sparse.second()(key)), bucket_count());
  }

  template<typename Other>
  [[nodiscard]] auto constrained_find(const Other &key, std::size_t bucket) {
    for (auto it = begin(bucket), last = end(bucket); it != last; ++it) {
      if (packed.second()(it->first, key)) {
        return begin() + static_cast<typename iterator::difference_type>(it.index());
      }
    }

    return end();
  }

  template<typename Other>
  [[nodiscard]] auto constrained_find(const Other &key, std::size_t bucket) const {
    for (auto it = cbegin(bucket), last = cend(bucket); it != last; ++it) {
      if (packed.second()(it->first, key)) {
        return cbegin() + static_cast<typename iterator::difference_type>(it.index());
      }
    }

    return cend();
  }

  template<typename Other, typename... Args>
  [[nodiscard]] auto insert_or_do_nothing(Other &&key, Args &&...args) {
    const auto index = key_to_bucket(key);

    if (auto it = constrained_find(key, index); it != end()) {
      return std::make_pair(it, false);
    }

    packed.first().emplace_back(sparse.first()[index],
                                std::piecewise_construct,
                                std::forward_as_tuple(std::forward<Other>(key)),
                                std::forward_as_tuple(std::forward<Args>(args)...));
    sparse.first()[index] = packed.first().size() - 1u;
    rehash_if_required();

    return std::make_pair(--end(), true);
  }

  template<typename Other, typename Arg>
  [[nodiscard]] auto insert_or_overwrite(Other &&key, Arg &&value) {
    const auto index = key_to_bucket(key);

    if (auto it = constrained_find(key, index); it != end()) {
      it->second = std::forward<Arg>(value);
      return std::make_pair(it, false);
    }

    packed.first().emplace_back(sparse.first()[index], std::forward<Other>(key), std::forward<Arg>(value));
    sparse.first()[index] = packed.first().size() - 1u;
    rehash_if_required();

    return std::make_pair(--end(), true);
  }

  void move_and_pop(const std::size_t pos) {
    if (const auto last = size() - 1u; pos != last) {
      size_type *curr = sparse.first().data() + key_to_bucket(packed.first().back().element.first);
      packed.first()[pos] = std::move(packed.first().back());
      for (; *curr != last; curr = &packed.first()[*curr].next) {}
      *curr = pos;
    }

    packed.first().pop_back();
  }

  void rehash_if_required() {
    if (size() > (bucket_count() * max_load_factor())) {
      rehash(bucket_count() * 2u);
    }
  }

 public:
  /*! @brief Key type of the container. */
  using key_type = Key;
  /*! @brief Mapped type of the container. */
  using mapped_type = Type;
  /*! @brief Key-value type of the container. */
  using value_type = std::pair<const Key, Type>;
  /*! @brief Unsigned integer type. */
  using size_type = std::size_t;
  /*! @brief Type of function to use to hash the keys. */
  using hasher = Hash;
  /*! @brief Type of function to use to compare the keys for equality. */
  using key_equal = KeyEqual;
  /*! @brief Allocator type. */
  using allocator_type = Allocator;
  /*! @brief Input iterator type. */
  using iterator = internal::dense_map_iterator<typename packed_container_type::iterator>;
  /*! @brief Constant input iterator type. */
  using const_iterator = internal::dense_map_iterator<typename packed_container_type::const_iterator>;
  /*! @brief Input iterator type. */
  using local_iterator = internal::dense_map_local_iterator<typename packed_container_type::iterator>;
  /*! @brief Constant input iterator type. */
  using const_local_iterator = internal::dense_map_local_iterator<typename packed_container_type::const_iterator>;

  /*! @brief Default constructor. */
  dense_map()
      : dense_map{minimum_capacity} {}

  /**
   * @brief Constructs an empty container with a given allocator.
   * @param allocator The allocator to use.
   */
  explicit dense_map(const allocator_type &allocator)
      : dense_map{minimum_capacity, hasher{}, key_equal{}, allocator} {}

  /**
   * @brief Constructs an empty container with a given allocator and user
   * supplied minimal number of buckets.
   * @param cnt Minimal number of buckets.
   * @param allocator The allocator to use.
   */
  dense_map(const size_type cnt, const allocator_type &allocator)
      : dense_map{cnt, hasher{}, key_equal{}, allocator} {}

  /**
   * @brief Constructs an empty container with a given allocator, hash
   * function and user supplied minimal number of buckets.
   * @param cnt Minimal number of buckets.
   * @param hash Hash function to use.
   * @param allocator The allocator to use.
   */
  dense_map(const size_type cnt, const hasher &hash, const allocator_type &allocator)
      : dense_map{cnt, hash, key_equal{}, allocator} {}

  /**
   * @brief Constructs an empty container with a given allocator, hash
   * function, compare function and user supplied minimal number of buckets.
   * @param cnt Minimal number of buckets.
   * @param hash Hash function to use.
   * @param equal Compare function to use.
   * @param allocator The allocator to use.
   */
  explicit dense_map(const size_type cnt,
                     const hasher &hash = hasher{},
                     const key_equal &equal = key_equal{},
                     const allocator_type &allocator = allocator_type{})
      : sparse{allocator, hash},
        packed{allocator, equal},
        threshold{default_threshold} {
    rehash(cnt);
  }

  /*! @brief Default copy constructor. */
  dense_map(const dense_map &) = default;

  /**
   * @brief Allocator-extended copy constructor.
   * @param other The instance to copy from.
   * @param allocator The allocator to use.
   */
  dense_map(const dense_map &other, const allocator_type &allocator)
      : sparse{std::piecewise_construct, std::forward_as_tuple(other.sparse.first(), allocator),
               std::forward_as_tuple(other.sparse.second())},
        packed{std::piecewise_construct, std::forward_as_tuple(other.packed.first(), allocator),
               std::forward_as_tuple(other.packed.second())},
        threshold{other.threshold} {}

  /*! @brief Default move constructor. */
  dense_map(dense_map &&) noexcept(std::is_nothrow_move_constructible_v<compressed_pair<sparse_container_type, hasher>>
      && std::is_nothrow_move_constructible_v<compressed_pair<packed_container_type, key_equal>>) = default;

  /**
   * @brief Allocator-extended move constructor.
   * @param other The instance to move from.
   * @param allocator The allocator to use.
   */
  dense_map(dense_map &&other, const allocator_type &allocator)
      : sparse{std::piecewise_construct, std::forward_as_tuple(std::move(other.sparse.first()), allocator),
               std::forward_as_tuple(std::move(other.sparse.second()))},
        packed{std::piecewise_construct, std::forward_as_tuple(std::move(other.packed.first()), allocator),
               std::forward_as_tuple(std::move(other.packed.second()))},
        threshold{other.threshold} {}

  /**
   * @brief Default copy assignment operator.
   * @return This container.
   */
  dense_map &operator=(const dense_map &) = default;

  /**
   * @brief Default move assignment operator.
   * @return This container.
   */
  dense_map &operator=(dense_map &&) noexcept(
  std::is_nothrow_move_assignable_v<compressed_pair<sparse_container_type, hasher>>
      && std::is_nothrow_move_assignable_v<compressed_pair<packed_container_type, key_equal>>) = default;

  /**
   * @brief Returns the associated allocator.
   * @return The associated allocator.
   */
  [[nodiscard]] constexpr allocator_type get_allocator() const noexcept {
    return sparse.first().get_allocator();
  }

  /**
   * @brief Returns an iterator to the beginning.
   *
   * The returned iterator points to the first instance of the internal array.
   * If the array is empty, the returned iterator will be equal to `end()`.
   *
   * @return An iterator to the first instance of the internal array.
   */
  [[nodiscard]] const_iterator cbegin() const noexcept {
    return packed.first().begin();
  }

  /*! @copydoc cbegin */
  [[nodiscard]] const_iterator begin() const noexcept {
    return cbegin();
  }

  /*! @copydoc begin */
  [[nodiscard]] iterator begin() noexcept {
    return packed.first().begin();
  }

  /**
   * @brief Returns an iterator to the end.
   *
   * The returned iterator points to the element following the last instance
   * of the internal array. Attempting to dereference the returned iterator
   * results in undefined behavior.
   *
   * @return An iterator to the element following the last instance of the
   * internal array.
   */
  [[nodiscard]] const_iterator cend() const noexcept {
    return packed.first().end();
  }

  /*! @copydoc cend */
  [[nodiscard]] const_iterator end() const noexcept {
    return cend();
  }

  /*! @copydoc end */
  [[nodiscard]] iterator end() noexcept {
    return packed.first().end();
  }

  /**
   * @brief Checks whether a container is empty.
   * @return True if the container is empty, false otherwise.
   */
  [[nodiscard]] bool empty() const noexcept {
    return packed.first().empty();
  }

  /**
   * @brief Returns the number of elements in a container.
   * @return Number of elements in a container.
   */
  [[nodiscard]] size_type size() const noexcept {
    return packed.first().size();
  }

  /**
   * @brief Returns the maximum possible number of elements.
   * @return Maximum possible number of elements.
   */
  [[nodiscard]] size_type max_size() const noexcept {
    return packed.first().max_size();
  }

  /*! @brief Clears the container. */
  void clear() noexcept {
    sparse.first().clear();
    packed.first().clear();
    rehash(0u);
  }

  /**
   * @brief Inserts an element into the container, if the key does not exist.
   * @param value A key-value pair eventually convertible to the value type.
   * @return A pair consisting of an iterator to the inserted element (or to
   * the element that prevented the insertion) and a bool denoting whether the
   * insertion took place.
   */
  std::pair<iterator, bool> insert(const value_type &value) {
    return insert_or_do_nothing(value.first, value.second);
  }

  /*! @copydoc insert */
  std::pair<iterator, bool> insert(value_type &&value) {
    return insert_or_do_nothing(std::move(value.first), std::move(value.second));
  }

  /**
   * @copydoc insert
   * @tparam Arg Type of the key-value pair to insert into the container.
   */
  template<typename Arg>
  std::enable_if_t<std::is_constructible_v<value_type, Arg &&>, std::pair<iterator, bool>>
  insert(Arg &&value) {
    return insert_or_do_nothing(std::forward<Arg>(value).first, std::forward<Arg>(value).second);
  }

  /**
   * @brief Inserts elements into the container, if their keys do not exist.
   * @tparam It Type of input iterator.
   * @param first An iterator to the first element of the range of elements.
   * @param last An iterator past the last element of the range of elements.
   */
  template<typename It>
  void insert(It first, It last) {
    for (; first != last; ++first) {
      insert(*first);
    }
  }

  /**
   * @brief Inserts an element into the container or assigns to the current
   * element if the key already exists.
   * @tparam Arg Type of the value to insert or assign.
   * @param key A key used both to look up and to insert if not found.
   * @param value A value to insert or assign.
   * @return A pair consisting of an iterator to the element and a bool
   * denoting whether the insertion took place.
   */
  template<typename Arg>
  std::pair<iterator, bool> insert_or_assign(const key_type &key, Arg &&value) {
    return insert_or_overwrite(key, std::forward<Arg>(value));
  }

  /*! @copydoc insert_or_assign */
  template<typename Arg>
  std::pair<iterator, bool> insert_or_assign(key_type &&key, Arg &&value) {
    return insert_or_overwrite(std::move(key), std::forward<Arg>(value));
  }

  /**
   * @brief Constructs an element in-place, if the key does not exist.
   *
   * The element is also constructed when the container already has the key,
   * in which case the newly constructed object is destroyed immediately.
   *
   * @tparam Args Types of arguments to forward to the constructor of the
   * element.
   * @param args Arguments to forward to the constructor of the element.
   * @return A pair consisting of an iterator to the inserted element (or to
   * the element that prevented the insertion) and a bool denoting whether the
   * insertion took place.
   */
  template<typename... Args>
  std::pair<iterator, bool> emplace([[maybe_unused]] Args &&...args) {
    if constexpr (sizeof...(Args) == 0u) {
      return insert_or_do_nothing(key_type{});
    } else if constexpr (sizeof...(Args) == 1u) {
      return insert_or_do_nothing(std::forward<Args>(args).first..., std::forward<Args>(args).second...);
    } else if constexpr (sizeof...(Args) == 2u) {
      return insert_or_do_nothing(std::forward<Args>(args)...);
    } else {
      auto &node = packed.first().emplace_back(packed.first().size(), std::forward<Args>(args)...);
      const auto index = key_to_bucket(node.element.first);

      if (auto it = constrained_find(node.element.first, index); it != end()) {
        packed.first().pop_back();
        return std::make_pair(it, false);
      }

      std::swap(node.next, sparse.first()[index]);
      rehash_if_required();

      return std::make_pair(--end(), true);
    }
  }

  /**
   * @brief Inserts in-place if the key does not exist, does nothing if the
   * key exists.
   * @tparam Args Types of arguments to forward to the constructor of the
   * element.
   * @param key A key used both to look up and to insert if not found.
   * @param args Arguments to forward to the constructor of the element.
   * @return A pair consisting of an iterator to the inserted element (or to
   * the element that prevented the insertion) and a bool denoting whether the
   * insertion took place.
   */
  template<typename... Args>
  std::pair<iterator, bool> try_emplace(const key_type &key, Args &&...args) {
    return insert_or_do_nothing(key, std::forward<Args>(args)...);
  }

  /*! @copydoc try_emplace */
  template<typename... Args>
  std::pair<iterator, bool> try_emplace(key_type &&key, Args &&...args) {
    return insert_or_do_nothing(std::move(key), std::forward<Args>(args)...);
  }

  /**
   * @brief Removes an element from a given position.
   * @param pos An iterator to the element to remove.
   * @return An iterator following the removed element.
   */
  iterator erase(const_iterator pos) {
    const auto diff = pos - cbegin();
    erase(pos->first);
    return begin() + diff;
  }

  /**
   * @brief Removes the given elements from a container.
   * @param first An iterator to the first element of the range of elements.
   * @param last An iterator past the last element of the range of elements.
   * @return An iterator following the last removed element.
   */
  iterator erase(const_iterator first, const_iterator last) {
    const auto dist = first - cbegin();

    for (auto from = last - cbegin(); from != dist; --from) {
      erase(packed.first()[from - 1u].element.first);
    }

    return (begin() + dist);
  }

  /**
   * @brief Removes the element associated with a given key.
   * @param key A key value of an element to remove.
   * @return Number of elements removed (either 0 or 1).
   */
  size_type erase(const key_type &key) {
    for (size_type *curr = sparse.first().data() + key_to_bucket(key); *curr != (std::numeric_limits<size_type>::max)();
         curr = &packed.first()[*curr].next) {
      if (packed.second()(packed.first()[*curr].element.first, key)) {
        const auto index = *curr;
        *curr = packed.first()[*curr].next;
        move_and_pop(index);
        return 1u;
      }
    }

    return 0u;
  }

  /**
   * @brief Exchanges the contents with those of a given container.
   * @param other Container to exchange the content with.
   */
  void swap(dense_map &other) {
    using std::swap;
    swap(sparse, other.sparse);
    swap(packed, other.packed);
    swap(threshold, other.threshold);
  }

  /**
   * @brief Accesses a given element with bounds checking.
   * @param key A key of an element to find.
   * @return A reference to the mapped value of the requested element.
   */
  [[nodiscard]] mapped_type &at(const key_type &key) {
    auto it = find(key);
    RD_ASSERT(it != end(), "Invalid key");
    return it->second;
  }

  /*! @copydoc at */
  [[nodiscard]] const mapped_type &at(const key_type &key) const {
    auto it = find(key);
    RD_ASSERT(it != cend(), "Invalid key");
    return it->second;
  }

  /**
   * @brief Accesses or inserts a given element.
   * @param key A key of an element to find or insert.
   * @return A reference to the mapped value of the requested element.
   */
  [[nodiscard]] mapped_type &operator[](const key_type &key) {
    return insert_or_do_nothing(key).first->second;
  }

  /**
   * @brief Accesses or inserts a given element.
   * @param key A key of an element to find or insert.
   * @return A reference to the mapped value of the requested element.
   */
  [[nodiscard]] mapped_type &operator[](key_type &&key) {
    return insert_or_do_nothing(std::move(key)).first->second;
  }

  /**
   * @brief Returns the number of elements matching a key (either 1 or 0).
   * @param key Key value of an element to search for.
   * @return Number of elements matching the key (either 1 or 0).
   */
  [[nodiscard]] size_type count(const key_type &key) const {
    return find(key) != end();
  }

  /**
   * @brief Returns the number of elements matching a key (either 1 or 0).
   * @tparam Other Type of the key value of an element to search for.
   * @param key Key value of an element to search for.
   * @return Number of elements matching the key (either 1 or 0).
   */
  template<typename Other>
  [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>,
                                 std::conditional_t<false, Other, size_type>>
  count(const Other &key) const {
    return find(key) != end();
  }

  /**
   * @brief Finds an element with a given key.
   * @param key Key value of an element to search for.
   * @return An iterator to an element with the given key. If no such element
   * is found, a past-the-end iterator is returned.
   */
  [[nodiscard]] iterator find(const key_type &key) {
    return constrained_find(key, key_to_bucket(key));
  }

  /*! @copydoc find */
  [[nodiscard]] const_iterator find(const key_type &key) const {
    return constrained_find(key, key_to_bucket(key));
  }

  /**
   * @brief Finds an element with a key that compares _equivalent_ to a given
   * key.
   * @tparam Other Type of the key value of an element to search for.
   * @param key Key value of an element to search for.
   * @return An iterator to an element with the given key. If no such element
   * is found, a past-the-end iterator is returned.
   */
  template<typename Other>
  [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>,
                                 std::conditional_t<false, Other, iterator>>
  find(const Other &key) {
    return constrained_find(key, key_to_bucket(key));
  }

  /*! @copydoc find */
  template<typename Other>
  [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>,
                                 std::conditional_t<false, Other, const_iterator>>
  find(const Other &key) const {
    return constrained_find(key, key_to_bucket(key));
  }

  /**
   * @brief Returns a range containing all elements with a given key.
   * @param key Key value of an element to search for.
   * @return A pair of iterators pointing to the first element and past the
   * last element of the range.
   */
  [[nodiscard]] std::pair<iterator, iterator> equal_range(const key_type &key) {
    const auto it = find(key);
    return {it, it + !(it == end())};
  }

  /*! @copydoc equal_range */
  [[nodiscard]] std::pair<const_iterator, const_iterator> equal_range(const key_type &key) const {
    const auto it = find(key);
    return {it, it + !(it == cend())};
  }

  /**
   * @brief Returns a range containing all elements that compare _equivalent_
   * to a given key.
   * @tparam Other Type of an element to search for.
   * @param key Key value of an element to search for.
   * @return A pair of iterators pointing to the first element and past the
   * last element of the range.
   */
  template<typename Other>
  [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>,
                                 std::conditional_t<false, Other, std::pair<iterator, iterator>>>
  equal_range(const Other &key) {
    const auto it = find(key);
    return {it, it + !(it == end())};
  }

  /*! @copydoc equal_range */
  template<typename Other>
  [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>,
                                 std::conditional_t<false, Other, std::pair<const_iterator, const_iterator>>>
  equal_range(const Other &key) const {
    const auto it = find(key);
    return {it, it + !(it == cend())};
  }

  /**
   * @brief Checks if the container contains an element with a given key.
   * @param key Key value of an element to search for.
   * @return True if there is such an element, false otherwise.
   */
  [[nodiscard]] bool contains(const key_type &key) const {
    return (find(key) != cend());
  }

  /**
   * @brief Checks if the container contains an element with a key that
   * compares _equivalent_ to a given value.
   * @tparam Other Type of the key value of an element to search for.
   * @param key Key value of an element to search for.
   * @return True if there is such an element, false otherwise.
   */
  template<typename Other>
  [[nodiscard]] std::enable_if_t<is_transparent_v<hasher> && is_transparent_v<key_equal>,
                                 std::conditional_t<false, Other, bool>>
  contains(const Other &key) const {
    return (find(key) != cend());
  }

  /**
   * @brief Returns an iterator to the beginning of a given bucket.
   * @param index An index of a bucket to access.
   * @return An iterator to the beginning of the given bucket.
   */
  [[nodiscard]] const_local_iterator cbegin(const size_type index) const {
    return {packed.first().begin(), sparse.first()[index]};
  }

  /**
   * @brief Returns an iterator to the beginning of a given bucket.
   * @param index An index of a bucket to access.
   * @return An iterator to the beginning of the given bucket.
   */
  [[nodiscard]] const_local_iterator begin(const size_type index) const {
    return cbegin(index);
  }

  /**
   * @brief Returns an iterator to the beginning of a given bucket.
   * @param index An index of a bucket to access.
   * @return An iterator to the beginning of the given bucket.
   */
  [[nodiscard]] local_iterator begin(const size_type index) {
    return {packed.first().begin(), sparse.first()[index]};
  }

  /**
   * @brief Returns an iterator to the end of a given bucket.
   * @param index An index of a bucket to access.
   * @return An iterator to the end of the given bucket.
   */
  [[nodiscard]] const_local_iterator cend([[maybe_unused]] const size_type index) const {
    return {packed.first().begin(), (std::numeric_limits<size_type>::max)()};
  }

  /**
   * @brief Returns an iterator to the end of a given bucket.
   * @param index An index of a bucket to access.
   * @return An iterator to the end of the given bucket.
   */
  [[nodiscard]] const_local_iterator end(const size_type index) const {
    return cend(index);
  }

  /**
   * @brief Returns an iterator to the end of a given bucket.
   * @param index An index of a bucket to access.
   * @return An iterator to the end of the given bucket.
   */
  [[nodiscard]] local_iterator end([[maybe_unused]] const size_type index) {
    return {packed.first().begin(), (std::numeric_limits<size_type>::max)()};
  }

  /**
   * @brief Returns the number of buckets.
   * @return The number of buckets.
   */
  [[nodiscard]] size_type bucket_count() const {
    return sparse.first().size();
  }

  /**
   * @brief Returns the maximum number of buckets.
   * @return The maximum number of buckets.
   */
  [[nodiscard]] size_type max_bucket_count() const {
    return sparse.first().max_size();
  }

  /**
   * @brief Returns the number of elements in a given bucket.
   * @param index The index of the bucket to examine.
   * @return The number of elements in the given bucket.
   */
  [[nodiscard]] size_type bucket_size(const size_type index) const {
    return static_cast<size_type>(std::distance(begin(index), end(index)));
  }

  /**
   * @brief Returns the bucket for a given key.
   * @param key The value of the key to examine.
   * @return The bucket for the given key.
   */
  [[nodiscard]] size_type bucket(const key_type &key) const {
    return key_to_bucket(key);
  }

  /**
   * @brief Returns the average number of elements per bucket.
   * @return The average number of elements per bucket.
   */
  [[nodiscard]] float load_factor() const {
    return size() / static_cast<float>(bucket_count());
  }

  /**
   * @brief Returns the maximum average number of elements per bucket.
   * @return The maximum average number of elements per bucket.
   */
  [[nodiscard]] float max_load_factor() const {
    return threshold;
  }

  /**
   * @brief Sets the desired maximum average number of elements per bucket.
   * @param value A desired maximum average number of elements per bucket.
   */
  void max_load_factor(const float value) {
    RD_ASSERT(value > 0.f, "Invalid load factor");
    threshold = value;
    rehash(0u);
  }

  /**
   * @brief Reserves at least the specified number of buckets and regenerates
   * the hash table.
   * @param cnt New number of buckets.
   */
  void rehash(const size_type cnt) {
    auto value = cnt > minimum_capacity ? cnt : minimum_capacity;
    const auto cap = static_cast<size_type>(size() / max_load_factor());
    value = value > cap ? value : cap;

    if (const auto sz = next_power_of_two(value); sz != bucket_count()) {
      sparse.first().resize(sz);

      for (auto &&elem : sparse.first()) {
        elem = std::numeric_limits<size_type>::max();
      }

      for (size_type pos{}, last = size(); pos < last; ++pos) {
        const auto index = key_to_bucket(packed.first()[pos].element.first);
        packed.first()[pos].next = std::exchange(sparse.first()[index], pos);
      }
    }
  }

  /**
   * @brief Reserves space for at least the specified number of elements and
   * regenerates the hash table.
   * @param cnt New number of elements.
   */
  void reserve(const size_type cnt) {
    packed.first().reserve(cnt);
    rehash(static_cast<size_type>(std::ceil(cnt / max_load_factor())));
  }

  /**
   * @brief Returns the function used to hash the keys.
   * @return The function used to hash the keys.
   */
  [[nodiscard]] hasher hash_function() const {
    return sparse.second();
  }

  /**
   * @brief Returns the function used to compare keys for equality.
   * @return The function used to compare keys for equality.
   */
  [[nodiscard]] key_equal key_eq() const {
    return packed.second();
  }

 private:
  compressed_pair<sparse_container_type, hasher> sparse;
  compressed_pair<packed_container_type, key_equal> packed;
  float threshold;
};

} // namespace rendu

namespace std {

template<typename Key, typename Value, typename Allocator>
struct uses_allocator<rendu::internal::dense_map_node<Key, Value>, Allocator>
    : std::true_type {
};

} // namespace std
#endif //RENDU_CORE_CONTAINER_DENSE_MAP_H_
